1. Technical Field
This invention relates generally to a thermal protection system for an output stage of an amplifier, and more particularly to the protection of the output devices of an output stage of an amplifier.
2. Related Art
Various circuit designs have been used to solve the problem of excessive localized heat production in output devices. Excessive heat leads to eventual breakdown and destruction of output devices. One popular circuit design is the V-I limiter. The V-I limiter circuit senses power dissipation in a output device by continually sensing the voltage and current of the output device, and limiting the output device driving signal when the product of the voltage and current exceeds a predetermined threshold. V-I limiter circuits are described in U.S. Pat. No. 3,234,453 issued to Klees, U.S. Pat. No. 3,500,218 issued to Burwen, U.S. Pat. No. 3,526,846 issued to Cambell, U.S. Pat. No. 3,536,958 issued to Sondermeyer, and U.S. Pat. No. 3,493,879 issued to Stanley.
The primary advantage of implementing a V-I limiter is simplicity. No additional power supplies are required to support the circuitry, and the circuit topology is relatively simple. The design of V-I limiters, however, typically sacrifices output power in order to protect the devices in “worst case” operating conditions that do not occur very often during normal operation. V-I limiters do not compensate for the actual temperature of the output devices being protected, but instead are usually designed with the assumption that the ambient temperature is high. Additionally, V-I limiters do not compensate for the actual power supply voltage, but instead are usually designed with the assumption of high power supply voltages. Such over-design results in increased costs due to extra output devices and mountings that are required.
Because of the limitations of V-I limiters, protection circuits were developed that simulate the actual temperature of an output device. These circuits are known as Junction Temperature Simulators (JTS). A JTS protection circuit is described in U.S. Pat. No. 4,330,809 issued to Stanley, and is incorporated by reference.
JTS circuits operate by monitoring the instantaneous power dissipated by an output device over time, and continuously calculating a differential in temperature between the output device and its heatsink. The instantaneous heatsink temperature is simultaneously measured, and the temperature differential and the heatsink temperature are summed to yield the simulated real-time output device temperature.
JTS protection circuits typically include an output limiter that monitors the simulated output device temperature during operation. When a threshold simulated temperature is reached, the driving signal to the output device is limited by the output limiter. JTS protection circuits maximize amplifier output by automatically reducing the output only when the prescribed maximum output device temperature is reached.
Traditionally, JTS has only been used in amplifiers rated at more than 200 watts because of constraints placed on the design of the circuits being protected and the associated costs. In traditional JTS circuit designs, the simulation circuitry is ground-referenced and powered by a regulated power supply having +/−15VDC rails. In amplifiers implementing the JTS circuitry, however, the power stages are typically implemented with swinging (non-ground referenced) rails. Therefore, a multi-channel amplifier implementing JTS requires a separate power supply for each channel, as well as an additional regulated power supply for the ground-referenced JTS circuit.
Because of the requirement for a separate regulated power supply, it is cost prohibitive to implement JTS in a simpler, lower-cost half-bridge amplifier where the power supply rails are ground-referenced and often shared with other output stages (channels). Therefore, lower-cost amplifiers that include thermal protection typically use V-I limiter protection circuitry. It would be desirable to implement JTS protection circuitry without incurring the cost of an additional traditional regulated power supply that requires additional secondary windings on a power transformer.